1. Field of the Invention
The present invention generally relates to an industrial robot system including a three-dimensional visual sensor, and more particularly to a method of re-calibrating a three-dimensional visual sensor and a device for carrying out the re-calibration method.
2. Description of Related Art
Three-dimensional visual sensors are widely used in various applications employing robot systems. Usage of three-dimensional visual sensors can be classified roughly into the following two cases (i) and (ii).
(i) A three-dimensional visual sensor is used with its sensor head mounted on a robot arm. The sensor head denotes a sensor section having an optical system (including an optical detector) for measuring an object, and control of the sensor head and processing of the detection signal are performed by a visual sensor controller. In this specification, the expression “mounting a three-dimensional visual sensor on a robot arm” is sometimes used in the sense of “mounting the sensor head of a three-dimensional visual sensor on a robot arm” in accordance with the practice of the technical field concerned.
(ii) A three-dimensional visual sensor is used with its sensor head set in a fixed position external to a robot.
In either case, it is well known that “calibration” is needed in order to perform measurement with proper accuracy by using a three-dimensional visual sensor. Calibration methods for three-dimensional visual sensors are described in detail in, for example, JP2690603B and JP2686351B and JP 08-5351A, JP08-233516A and JP08-272414A.
Thus, various calibration methods for three-dimensional visual sensors are conventionally known, but it is often the case that after proper measurement accuracy is once obtained by calibration, the measurement accuracy lowers, making accurate measurement impossible. Such a situation arises, in the aforementioned case (i), when the mounting position (including the orientation, as is the case with the following description) of the visual sensor (sensor head) mounted on the robot arm is displaced from the position where the calibration was performed. In the above case (ii), the same situation occurs when the set position of the visual sensor (sensor head) is displaced from the fixed position where the calibration was performed.
Such displacement of the sensor is caused, for example, when the visual sensor mounted on the robot arm collides against an object (including a person, as is the case with the following description) around the sensor, when the visual sensor is applied with an impact as a result of the collision of the robot arm against an object around the robot arm, or when an object collides with the sensor set in the fixed position. Also, even if the apparent position of the sensor is not displaced, the arrangement of the internal optical system may possibly change from that at the time of calibration due to an impact or the like, causing lowering of the measurement accuracy.
Conventionally, when such a situation occurs or is presumed to have occurred, identical calibration work is performed all over again. To again perform the identical calibration work, however, it is necessary that the relative positional relation between a measurement target and the visual sensor should be accurately reproduced by again using a calibration jig, requiring a heavy work load and inevitably prolonging the system downtime. The “measurement target” referred to in this specification denotes a target to be recognized by the visual sensor for the purpose of calibration, and a group of dots drawn on the calibration jig, for example, is used as the measurement target.